Feyman Diagram - Represent QED model(s)?

Hello - Trying the General Physics area. Question: Please explain how a Feyman Diagram represents or is conceived from a QED Model(s) i.e. IF it is the source of these diagrams? What is meant to be represented in them? Thank you

These are two sites that give good descriptions of what Feynman diagrams are. The latter is a bit more in depth:
http://www.physlink.com/Education/askexperts/ae263.cfm" [Broken]
http://en.wikipedia.org/wiki/Feynman_diagram" [Broken]

Also, here are some great videos where Feynman himself teaches QED. He describes the diagrams in the 3rd video:
http://www.vega.org.uk/video/subseries/8" [Broken]